Differential modulation of CaV2.3 Ca2+ channels by Gαq/11-coupled muscarinic receptors

Ca(V)2.3 subunits are expressed in neuronal and neuroendocrine cells where they are believed to form native R-type Ca2+ channels. Although R-type currents are involved in triggering neurotransmitter and hormone secretion, little is known about their modulation. Previous studies have shown that muscarinic acetylcholine receptors evoke both inhibition and stimulation of Ca(V)2.3. Muscarinic inhibition of Ca(V)2.3 is mediated by Gbetagamma subunits, whereas stimulation is mediated by pertussis toxin-insensitive Galpha subunits. In the present study, we compared modulation of Ca-V 2.3 by the three Galphaq/11-coupled muscarinic receptors (M1, M3, and M5). Our data indicate that these receptors trigger comparable stimulation of Ca-V 2.3. The signaling pathway that mediates stimulation was meticulously analyzed for M1 receptors. Stimulation is blocked by neutralizing antibodies directed against Galphaq/11, coexpression of the regulatory domain of protein kinase Cdelta (PKCdelta), preactivating PKC with phorbol ester, or pharmacological suppression of PKC with bisindolylmaleimide I. Stimulation of Ca-V 2.3 is Ca2+-independent and insensitive to 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13- methyl-5-oxo-5H-indolo(2,3-a) pyrrolo(3,4-c)-carbazole (Go 6976), a specific inhibitor of Ca2+-dependent PKC isozymes. These results indicate that muscarinic stimulation of Ca-V 2.3 involves signaling by Galphaq/11, diacylglycerol, and a Ca2+-independent PKC. In contrast to stimulation, the magnitude of Ca-V 2.3 inhibition depended on receptor subtype, with M3 and M5 receptors producing much larger Ca-V 2.3 inhibition than M1 receptors. Interestingly, muscarinic inhibition of Ca-V 2.3 was notably enhanced during pharmacological suppression of PKC, suggesting the presence of cross-talk between Gbetagamma-mediated inhibition and PKC-mediated stimulation of R-type channels similar to that described previously for N-type channels.